Let exceptions inherit standard exception classes with strings

[eyalroz]

The exception class inherits directly from std::exception. But - your exceptions seem to be either runtime errors (file not found), or invalid arguments (e.g. passing a null extension) etc. So, inherit from the appropriate standard exception class. That would also remove the need for the base class with the string member - don't reinvent the wheel unless you have to.

[eyalroz]

Actually, consider #32 as well - it's not clear you even want custom exceptions at all.